Outline Processor Markup Language (OPML) format

Introduction

The Outline Processor Markup Language (OPML format) is a widely used XML-based outline format designed to exchange structured lists of information. Originally developed for outlining documents, OPML has become the standard file format for managing RSS feeds, podcast subscriptions, and hierarchical data exchange. An OPML file (with the .opml extension) acts as a container that stores outlines in a structured, machine-readable format, making it easy to import, export, and share subscription lists across different platforms and applications.

Today, OPML is essential for RSS feed readers, podcast apps, knowledge management tools, and enterprise systems. Whether you need to open an OPML file, import OPML into Feedly or Inoreader, or export podcast subscriptions from Apple Podcasts, Overcast, Pocket Casts, or AntennaPod, this format ensures compatibility across multiple services. Its lightweight, flexible, and hierarchical data structure makes it especially valuable for businesses, developers, and professionals who work with structured content exchange.

In this guide, we’ll cover everything you need to know about the OPML format, including how to open, create, and convert OPML files, common OPML use cases, best tools, and best practices for working with OPML in 2025 and beyond.

What is OPML (Outline Processor Markup Language)?

OPML (Outline Processor Markup Language) is an XML-based file format designed to represent and exchange outlines in a structured way. An OPML file typically contains nested lists of items, making it useful for applications that rely on hierarchical data such as RSS feeds, podcast subscriptions, and digital outlines. Files saved with the .opml extension are lightweight, human-readable, and supported by a wide range of OPML readers, editors, and converters.

History and Evolution of the OPML File Format

The OPML format was originally developed in 2000 by UserLand Software as part of their outliner application. While it started as a tool for organizing documents and notes, OPML quickly gained traction in the early 2000s with the rise of RSS feed readers. It became the de facto standard for exporting and importing subscription lists between different platforms.

• 2000s – Adoption by early RSS readers and podcast managers.
• 2007 – Standardization of OPML 2.0 specification, introducing more flexibility.
• Present day – OPML remains widely used in podcast apps, news aggregators, enterprise knowledge management systems, and research tools.

Why OPML is Widely Used for RSS Feeds, Podcasts, and Outlines

The popularity of OPML comes from its simplicity and interoperability:

• RSS Feeds – Users can export all their subscribed feeds into one OPML file and import them into another reader (e.g., Feedly, Inoreader).
• Podcasts – Podcast apps like Apple Podcasts, Overcast, Pocket Casts, and AntennaPod use OPML to share entire subscription lists.
• Outlines – Writers, researchers, and businesses use OPML as a structured outline format for documents, reports, and project plans.

Its ability to act as a universal exchange format ensures that users are not locked into a single app or platform.

Importance of XML-Based Outline Formats in Structured Data Exchange

Being built on XML, OPML benefits from:

• Human- and machine-readable structure – easy to parse, edit, and validate.
• Cross-platform compatibility – works across different feed readers, podcast apps, and knowledge management tools.
• Scalability – supports everything from small outlines to enterprise-level structured data.
• Structured content exchange – essential for businesses and professionals who rely on data migration, knowledge sharing, and subscription management.

In short, OPML combines the strengths of XML’s structured markup with the flexibility of outlines, making it a long-standing standard for information portability and interoperability.

What is an OPML File?

An OPML file is a lightweight, text-based document saved with the .opml extension. Built on XML markup, it is primarily used to store and exchange hierarchical outlines, such as RSS feed subscriptions, podcast lists, or digital notes. Because OPML files are both machine-readable and human-readable, they are widely supported across different OPML readers, podcast apps, and feed aggregators.

Common uses of OPML files include:

• Exporting or importing subscription lists in RSS feed readers (e.g., Feedly, Inoreader).
• Sharing podcast subscriptions between apps like Apple Podcasts, Pocket Casts, Overcast, and AntennaPod.
• Organizing knowledge, research notes, and project outlines.

OPML Extension and OPML Schema 

• File Extension: OPML files use the .opml extension, similar to how XML files use .xml.
• Schema: The OPML schema defines the structure of the file. Each OPML file must contain:
  • A <head> section (metadata, such as title, owner name, creation date).
  • A <body> section (the main outline, containing hierarchical lists of <outline> elements).

This schema ensures that any OPML parser, viewer, or converter can correctly interpret the file’s structure, regardless of the application used to create it.

File Structure: Headers, Outlines, and Nested Items

An OPML file structure is simple yet powerful. It consists of:

• Headers (<head>)

1. • Contains metadata about the OPML file, such as:
     • Title of the outline
     • Owner name or email
     • Date of creation or modification
2. Outlines (<body> with <outline> elements)
   • Each <outline> represents a single entry, such as a podcast feed or RSS subscription.
   • Outlines can include attributes like text, type, and xmlUrl (for feed URLs).
3. Nested Items
   • OPML supports multiple levels of nested <outline> elements, allowing hierarchical organization of feeds, categories, or notes.
   • Example: A parent outline for “Podcasts” with nested outlines for different podcast categories.

OPML Format Use Cases

The OPML format has become the universal standard for exchanging structured subscription lists, outlines, and hierarchical data. Its compatibility across multiple applications makes it invaluable for RSS feed readers, podcast apps, and knowledge management systems. Below are the most common use cases of OPML in practice.

OPML for RSS Feed Subscriptions

One of the most common uses of OPML is in RSS feed management. Instead of manually subscribing to feeds one by one, users can:

• Export subscriptions – Save all subscribed RSS feeds from one reader into a single OPML file.
• Import subscriptions – Load that OPML file into another RSS reader to instantly restore the entire list.

This process is supported by popular platforms such as:

• Feedly – Import/export RSS subscriptions in OPML format for quick migration.
• Inoreader – Use OPML to back up and restore feeds or share subscriptions.
• Other readers like The Old Reader, NewsBlur, and Tiny Tiny RSS.

Pro Tip: Always keep a backup OPML file of your RSS feeds to ensure full requirements lifecycle coverage in case of migration or data loss.

OPML in Podcast Management

Podcast subscriptions are another major use case for OPML. Since many users listen to podcasts across multiple devices or apps, the OPML format enables seamless transfer of subscription lists:

• Export podcast subscriptions – Save your current podcast list to an OPML file.
• Import OPML into podcast apps – Restore or migrate subscriptions without needing to search for each feed manually.

Popular podcast platforms that support OPML include:

• Apple Podcasts – Import/export subscription lists.
• Overcast – OPML-based backup and restore functionality.
• AntennaPod – Open-source app supporting OPML import/export.
• Pocket Casts – Share and migrate subscriptions via OPML.

Pro Tip: When switching podcast apps, export your OPML file first to avoid losing your curated list of subscriptions.

OPML for Knowledge and Outline Sharing

Beyond feeds and podcasts, OPML is also used for knowledge organization and structured outline sharing. Its hierarchical data structure makes it ideal for:

• Outlining documents and research papers.
• Structuring project requirements or knowledge bases.
• Exchanging data between enterprise knowledge management tools.

For researchers, writers, and businesses, OPML provides a flexible way to create, share, and collaborate on structured information without format lock-in.

Pro Tip: Use OPML as a reusable requirements component in enterprise projects, enabling better requirements reusability and traceability across systems.

OPML vs Other Formats

While the OPML format is widely used for exchanging subscription lists and outlines, it is often compared with other structured data formats like XML, RSS, and Atom. Understanding these differences helps determine when OPML is the most effective choice.

OPML vs XML

• XML (Extensible Markup Language) is a general-purpose markup language for structuring data.
• OPML (Outline Processor Markup Language) is a specialized XML-based outline format designed specifically for hierarchical lists.

Key Difference: All OPML files are XML, but not all XML files are OPML. XML is flexible and broad, while OPML provides a defined schema tailored to outlines, RSS feeds, and podcast subscriptions.

Best Use Case: Use OPML when you need to share structured outlines (e.g., feed lists), and XML when you require a custom schema for broader data exchange.

OPML vs RSS vs Atom

• RSS (Really Simple Syndication) and Atom are formats used to syndicate web content like blog posts or news updates.
• OPML does not deliver content itself but stores lists of RSS or Atom feeds for easy import/export.

Comparison:

• RSS/Atom – Contain live content updates.
• OPML – Contains collections of RSS/Atom feed URLs organized in a structured outline.

Best Use Case: Use RSS or Atom to access ongoing updates from a single source; use OPML to manage and migrate entire feed collections.

Advantages and Limitations of OPML Compared to Alternatives

Advantages of OPML:

• Simple, lightweight, and human-readable.
• Universally supported by most feed readers and podcast apps (Feedly, Inoreader, Apple Podcasts, Pocket Casts, Overcast, AntennaPod).
• Ideal for structured content exchange and subscription migration.
• Supports hierarchical data organization with nested outlines.

Limitations of OPML:

• Not designed for delivering live updates (unlike RSS or Atom).
• Limited schema flexibility compared to raw XML.
• Relies on application support, not every platform fully supports OPML import/export.

Pro Tip: Use OPML as a complement to RSS and Atom. Manage your subscription lists with OPML, but rely on RSS/Atom feeds for actual content delivery.

Advanced OPML Usage for Developers

While OPML is best known for RSS feed subscriptions and podcast management, developers and enterprises use it in more advanced ways. The OPML 2.0 specification, open-source tools on GitHub, and dedicated APIs and libraries make it a versatile format for custom enterprise and IT applications.

OPML 2.0 Specification Explained

The OPML 2.0 specification (an update to the original OPML 1.0) defines a more flexible and standardized structure for OPML files. Key features include:

• Backward compatibility with OPML 1.0.
• Extended attributes for <outline> elements (e.g., text, type, xmlUrl, htmlUrl).
• Improved support for hierarchical outlines and nested items.
• Broader use cases beyond RSS and podcasts, including enterprise outlines and structured documentation.

Pro Tip: Always validate OPML files against the OPML 2.0 specification to ensure compatibility across different readers and applications.

GitHub Resources for OPML Format

Developers can leverage GitHub repositories for:

• OPML parsers in multiple programming languages (Python, Java, JavaScript, PHP).
• OPML-to-JSON/XML/CSV converters.
• Open-source OPML editors and viewers.
• Sample OPML files and templates for testing.

Popular GitHub projects provide tools for automation, parsing, and validation, ensuring smooth integration into custom workflows.

APIs and Libraries to Parse and Process OPML

To automate OPML handling, developers often use:

• OPML parser APIs for importing/exporting subscription lists.
• Libraries (e.g., Python opml package, Node.js opmlparser, PHP simplexml) to read and manipulate OPML structures.
• Integration with content management systems (CMS) and knowledge management platforms for structured data exchange.

Pro Tip: Use OPML parsing libraries with built-in XML validation to prevent issues from malformed or corrupted OPML files.

Custom Applications of OPML in Enterprise and IT Systems

Beyond media subscriptions, enterprises use OPML for:

• Knowledge management – Sharing structured outlines across departments.
• Requirements engineering – Storing and reusing hierarchical requirements data.
• Project documentation – Importing/exporting outlines in IT project tools.
• Cross-platform data migration – Moving structured content between enterprise systems.

In industries like aerospace, healthcare, and IT consulting, OPML helps manage structured content exchange with traceability and reusability.

Common Issues and Troubleshooting with OPML Files

While the OPML format is lightweight and simple, users often encounter problems when working with RSS readers, podcast apps, or OPML editors. Below are the most common issues, their causes, and practical fixes.

Why an OPML File Won’t Open

An OPML file may fail to open due to:

• Unsupported application – Not all apps accept OPML imports (e.g., some podcast apps may require a specific schema).
• Wrong file extension – The file must have the .opml extension; renaming without proper formatting can break compatibility.
• Encoding errors – Non-UTF-8 encoded OPML files may not load in some readers.

Pro Tip: Always open OPML files with a dedicated OPML viewer or OPML editor to confirm the file is structured correctly before importing into another application.

Fixing Invalid or Corrupted OPML Files

If an OPML file is invalid or corrupted, common solutions include:

• Validate the file – Use an XML validator or OPML-specific schema checker to identify broken tags.
• Repair encoding issues – Convert the file to UTF-8 encoding.
• Manual fixes – Open the OPML in a text editor and correct missing or mismatched <outline> or <head> tags.
• Use a converter – Convert OPML to XML/JSON and back to regenerate a valid structure.

Pro Tip: Keep backup OPML files of your subscriptions. This ensures recovery if one file becomes corrupted.

Common Mistakes When Using OPML with RSS Readers and Podcast Apps

Even valid OPML files may fail to work as expected due to user mistakes:

• Duplicate subscriptions – Importing the same OPML multiple times can create duplicates.
• Unsupported tags – Some apps ignore advanced attributes from OPML 2.0.
• Missing xmlURL attributes – Without valid feed URLs, readers cannot fetch RSS or podcast content.
• App-specific limitations – Some podcast apps (e.g., older versions of Apple Podcasts) only support partial OPML import.

Pro Tip: Before importing, review the OPML file in a text editor to confirm that each <outline> has a valid xmlUrl. This prevents broken feeds in your reader.

Best Practices for Working with OPML Files

To get the most out of the Outline Processor Markup Language (OPML) format, it’s important to follow best practices that ensure compatibility, reliability, and efficiency when sharing or importing files across platforms.

Maintaining Live Subscription Lists

A common mistake users make is treating OPML as a static file. Instead, advanced users and organizations maintain live OPML subscription lists that update automatically when new feeds are added.

• Use a centralized OPML generator or feed aggregator to keep lists current.
• Share live OPML URLs instead of static .opml files for dynamic updates.
• Ideal for teams, enterprises, and podcast networks managing multiple subscriptions.

Pro Tip: Tools like Feedly, Inoreader, and GitHub OPML repositories allow you to maintain and share live, version-controlled OPML lists.

Ensuring Compatibility Across Different OPML Readers

Not all RSS readers, podcast apps, or OPML editors interpret OPML files the same way. Compatibility issues can lead to missing feeds or failed imports.

• Stick to the OPML 2.0 specification for broader support.
• Validate OPML files using XML schema validators before sharing.
• Test imports across multiple apps (e.g., Apple Podcasts, Overcast, Pocket Casts, Feedly, Inoreader) to ensure cross-platform usability.

Pro Tip: Avoid using app-specific attributes in OPML files unless the audience is locked into that ecosystem.

Regular Backups of OPML Files for Podcasts and Feeds

Since OPML files act as a portable subscription archive, losing them means losing your curated feed list.

• Export your RSS subscriptions and podcast feeds regularly to OPML.
• Store backups in cloud storage (Google Drive, Dropbox, OneDrive) or GitHub.
• Version control OPML files to track changes in subscriptions.

Pro Tip: Set a monthly reminder to export your OPML file, especially if you frequently add or remove feeds. This ensures data portability and recovery in case of app crashes or migration.

By following these best practices, users ensure that their OPML files remain valid, portable, and compatible across different readers and platforms, making it easier to manage subscriptions and outlines.

Conclusion

The Outline Processor Markup Language (OPML) format remains one of the most practical and lightweight standards for managing RSS feeds, podcast subscriptions, and structured outlines. Its XML-based design, portability, and compatibility across platforms make it a go-to choice for individuals, businesses, and developers who need efficient data exchange and subscription management.

By understanding the history of OPML, its file structure, key use cases, best practices, and troubleshooting steps, users can maximize its value while avoiding common pitfalls like corrupted files, incompatible readers, or outdated subscription lists. Whether you are a casual podcast listener, a power RSS user, or a developer integrating OPML into enterprise systems, adopting standardized practices and regular backups ensures smooth and reliable usage.

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